That picture does not tell the cause of why bootloader installation failed. But there is a way to get the logs that would tell us the cause. Boot the ISO Live, open Konsole (terminal), and use this copy and paste command:
$ pkexec calamares -d | tee /home/live/calamares-installation-log.txt
That will create the file in home directory called calamares-installation-log.txt which you can attach here. Then we can probably learn what the cause of this problem is.
For any user having difficulties installing OMLx Rock or ROME versions there is a what to do guide here.